The Fairmont is the old Railroad Hotel, isn't it? That is tempting on a lot of levels, but for the $210+ price tag.
As much as I'd like to be by a nice mainline (and I'm not even kidding there), my only concern about an intervening link is turning the trip into a Charlie Foxtrot. There does seem to be a Holiday Inn downtown that I can snag for $99, and it's only about a 2/3 of a mile from Central Station on foot (which is nice...I don't expect to have enough baggage to merit a cab, though I'll take one on advice if downtown is particularly bad at night...or it's also only two stops on the metro, which is also nice and avoids a walk). That's...about 40% less than what I was expecting to see at best, based on my experience in NYC, Pittsburgh, etc.
How is the commuter rail in Montreal vs. the subway, and how is the reliability (and the crowding, considering how far up the line Dorval is)? I get nervous around lower-frequency commuter lines (particularly packed ones...for example, in a vacuum I'm not sure that I would trust the VRE for a one-off trip to Washington that involved a connection without a fairly long layover, though for regular service that I was used to only requiring me to go downtown I'd be more open to it.